Stiffening of matter in quark-hadron continuity: a mini-review

TL;DR

Recent neutron star observations suggest a rapid stiffening of dense QCD matter above nuclear saturation density, indicating early quark matter formation and continuous hadron-quark transition within a quarkyonic matter framework.

Contribution

This paper introduces the IdylliQ model, allowing smooth transition between baryonic and quark descriptions, and discusses the implications of quarkyonic matter for neutron star physics.

Findings

Quark matter forms at 2-3 times nuclear saturation density.

Continuous hadron-quark transition is modeled within quarkyonic matter.

The IdylliQ model provides analytic insights into the transition regime.

Abstract

Recent observations of neutron stars, combined with causality, thermodynamic stability, and nuclear constraints, indicate a rapid stiffening of QCD matter at densities slightly above nuclear saturation density ($n_0 \simeq 0.16\,{\rm fm}^{-3}$). The evolution of the stiffening is faster than expected from purely nucleonic models with many-body repulsion. Taking into account the quark substructure of baryons, we argue that the saturation of quark states occurs at $\sim$ 2-3$n_0$, driving quark matter formation even before baryonic cores of radius $\sim$0.5 fm spatially overlap. We describe the continuous transitions from hadronic to quark matter within a quarkyonic matter model in which gluons are assumed to remain confining at densities of interest. To obtain analytic insight into the transient regime, we construct an ideal model of quarkyonic matter, the {\it IdylliQ} model, in which one can freely switch from baryonic to quark descriptions and vice versa.